The prior art has often assembled optical systems for superimposing background and foreground visual information for giving the impression that performers are located at exotic locations that are actually provided only by slides or illuminated photographs.
Half-silvered mirrors have often been employed to accomplish the actual combination of the images. Alternately, the second (the background) scene is astigmatically predistorted so that an inclined mirror is capable of directing the second scene to an image-receiving surface without optically interfering with the light rays of the main image.
At other times the auxiliary object, which may be a counter register of numerals is simply placed in the main field of view at one side and is imaged by an objective lens as a part of the main field of view.
Still another arrangement employs a second lens that is moved into the optical path of the first lens when an auxiliary image is to be impressed upon an image-receiving surface, and then the second lens is moved out of the optical path of the first lens when the normal field-of-view is to be imaged upon the image-receiving surface.
A further arrangement has a very narrow auxiliary object, an incandescent filament, and a non-collimating mirror to pass light from the filament through a common objective lens to form a simple line image assymmetrically located with respect to the main image.